Some embodiments relate to a magnetoresistive device, and in more concrete terms, to a magnetoresistive device whose switching efficiency (κ) is high.
A magnetoresistive device (hereinafter simply termed an MR device) fundamentally has a three-layered structure, which is constituted with a free layer, a fixed layer, and a non-magnetic layer disposed between those two layers. There are two types of MR devices: one that exploits the giant magnetoresistance effect (GMR: Giant Magnetoresistance Effect), and the other that exploits the tunnel magnetoresistance effect (TMR: Tunnel Magnetoresistance Effect). With both of these types of MR devices, the orientations of magnetization of the free layer and the fixed layer being parallel (hereinafter this will be termed the parallel configuration) or being anti-parallel (hereinafter this will be termed the anti-parallel configuration), corresponds to either of “0” and “1” of a digital signal. It is expected that MR devices will be implemented in practice as storage cells or unit cells in an MRAM (magnetoresistive random access memory) which will be an alternative to the DRAMs and SRAMs that are currently in wide use. Since MR devices can be miniaturized, it is expected that they will be put into practical use as memory cells for gigabit class non-volatile memories.
Writing information into an MR device is performed by applying a current (i.e. a current pulse) in the direction perpendicular to the three layers of the MR device. When current is flowed to the MR device in this manner, spin transfer torque (STT) acts upon the free layer. By changing the direction in which the current flows, the direction of the free layer is reversed but the direction of the fixed layer is not reversed. In this way, by changing the direction in which the current flows, the magnetization configuration of the free layer and the fixed layer is changed from the parallel configuration to the anti-parallel configuration, or conversely is changed from the anti-parallel configuration to the parallel configuration. Reading out information from the MR device is performed by utilizing the fact that there is a difference in the magnitude of the magnetic resistance (MR) between the free layer and the fixed layer in the parallel configuration and in the anti-parallel configuration (i.e., the magnetoresistance effect). The magnetoresistance effect is evaluated by the MR ratio, and the MR ratio is defined as MR ratio=[(RAP−RP)/RP]×100(%). Here, RAP and RP represent the respective resistances between the free layer and the fixed layer in the anti-parallel configuration and in the parallel configuration. When a current is applied so as to flow in the direction perpendicular to the MR device, the voltage between the free layer and the fixed layer differs depending on whether the magnetization configuration of the free layer and the fixed layer is in the parallel configuration or in the anti-parallel configuration. Of course, the current for reading out information from the MR device is smaller than the current for writing information into the MR device.
While, in the initially produced MR devices, an easy magnetization directions of the free layer and the fixed layer were in-plane (in-plane type), in improved MR devices which have been produced thereafter, an easy magnetization directions of the free layer and the fixed layer were in perpendicular to the plane (perpendicular type). In both the in-plane type MR devices and the perpendicular type MR devices, the easy magnetization directions of the free layer and the fixed layer are the same. In other words, if the easy magnetization direction of the free layer is in-plane, then the easy magnetization direction of the fixed layer is also in-plane. If the easy magnetization direction of the free layer is in perpendicular, then the easy magnetization direction of the fixed layer is also in perpendicular. An MR device in which the easy magnetization directions of the free layer and of the fixed layer are the same is termed a collinear MR device. In this case, “collinear” refers to the state in which the easy magnetization directions are the same when the bias current flowing through the MR device is zero. Here, the state in which the bias current is zero means the state in which no current flows including write current and read current. It should be understood that, since MR devices are usually collinear, they are not particularly referred to as “collinear MR devices” but are normally referred to simply as “MR devices”. The threshold current ISW of the free layer is defined as the current that makes the energy barrier height zero. The threshold current ISW may be the minimum value (i.e. the threshold value) of current value needed for writing at absolute zero. Since the power consumption of the MR device depends upon ISW, it is desirable for ISW to be as small as possible. The threshold current ISW of a perpendicular type MR device is proportional to an effective first-order anisotropy constant Ku1,eff, and can be represented by the following expression (A):ISW=4(e·V/h−)(α/P)Ku1,eff.  (A)Here, e is the elementary electrical charge, V is the volume of the free layer, h− is the Dirac constant, α is the Gilbert damping constant of the free layer, and P (=0˜1) is the spin polarization of the current.
Next, a thermal stability factor of the MR device will be explained. The thermal stability factor Δ0 is the ratio of the magnetic anisotropy energy of the free layer to the thermal energy. The greater Δ0 is, the harder it becomes to erase the information “0” or “1”, since the magnetization in the free layer becomes more stable. In order for the MR device to be used as a memory cell in a gigabit class non-volatile memory that has a data retention time of ten years or longer, the thermal stability factor Δ0 is required to be 60 or greater. The Δ0 of a perpendicular type MR device is proportional to the first order effective anisotropy constant Ku1,eff, and can be represented by the following expression (B):Δ0=Ku1,effV/(kBT).  (B)Here, kB is Boltzmann's constant, and T is the temperature (unit: Kelvin). From expression (B), it will be understood that it is necessary to make Ku1,eff large in order to make Δ0 large. As is apparent from expression (A), however, when Ku1,eff is made large, ISW also becomes large. Therefore, with respect to a perpendicular type MR device, hitherto it has been considered to be impossible to achieve both of small ISW and large Δ0 by adjusting the anisotropy constant.
In general, spin torque switching efficiency (κ) is used as an index for showing the degree of coexistence with small ISW and large Δ0. Spin torque switching efficiency κ is defined as κ=Δ0/ISW, and the larger κ is, the better is the coexistence of small ISW and large Δ0. As will be described hereinafter, the second order term (Ku2) among the uniaxial anisotropy which is expressed with the constants (Ku1,eff, Ku2) has been ignored since Ku2 has been regarded as comparatively small. Ku2 has been regarded as zero in the setting of the specification for a related art perpendicular type MR device. In such case, the spin torque switching efficiency (κ) has been particularly expressed as κ(p0), and κ(p0) has been given by the following expression (C):κ=κ(p0)=[h−/(4e kBT)]×(P/α).  (C)Here, h− is the Dirac constant, e is the elementary electrical charge (a constant), kB is the Boltzmann constant, and T is the temperature (unit: Kelvin). T is assumed to be substantially a fixed value near room temperature. From expression (C), in order to make κ(p0) high, it is necessary to make P (the spin polarization) large, and/or to make α (the Gilbert damping constant of the free layer) small. Various types of materials and techniques for accomplishing high κ(p0), for example, been proposed in E. Kitagawa et al., IEDM Tech. Dig. (2012), pp. 29.4.1 through 29.4.4. However, the spin torque switching efficiency κ(p0) has not been greatly increased. This is also the case for in-plane type MR devices.